This chapter addresses part of the first question that is raised in the modeling process (Sect. 12.4.1): "What can I hope to achieve by modeling my bioreactor and what effort am I willing to put in to achieve it"? Specifically, it explores the question of how decisions made about which phenomena should be described by the model will affect the complexity of the model and the difficulty of its solution.
The system within an SSF bioreactor is so complex that any attempt to describe it in full detail will lead to a highly complex model that will need much experimental work to determine the parameters and may require long solution times. We argue that there is much to gain from using "fast-solving" models that recognize the heterogeneity within the substrate bed at the macroscale, but beyond this, take a relatively simple view of the system. This is not to say that more advanced bioreactor models would not be useful tools, just that they need several years of development before they will be available as easy-to-use software packages.
It is convenient to consider the question in two parts. Firstly, what level of detail should be used to describe the growth kinetics in the kinetic sub-model? Secondly, what is the appropriate level for describing transport process in the balance/transport sub-model? The following sections explore these questions.
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